The disclosed invention relates to ink jet printing devices, and more particularly to techniques for achieving improved print quality on print media that is susceptible to print quality degradation due to ink drop placement misalignment.
An ink jet printer forms a printed image by printing a pattern of individual dots at particular locations of a pixel array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes called "dot locations," "dot positions," or "pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Ink jet printers print dots by ejecting very small drops of ink onto the print medium, and commonly include a movable print carriage that supports one or more printheads each having ink ejecting nozzles. The print carriage traverses across the print medium along a carriage scan axis, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
Insofar as there is relative motion along the carriage scan axis between the ink jet nozzles and the print medium as drops are being ejected, the actual placement of a drop on the print medium depends on the relative scan velocity and the ejection velocity of a drop (also called drop velocity or vertical velocity in those implementations wherein the nozzles are directed downwardly) which is generally orthogonal to the carriage scan axis. The carriage scan velocity can be accurately controlled, and the actual drop placement depends predominantly on drop velocity. A drop having a higher than nominal drop velocity will have a shorter flight time than a drop having a nominal drop velocity. Since an ejected drop has a carriage scan axis velocity imparted by the relative motion between the nozzles and the print medium, as referenced to the print media, a drop of higher than nominal drop velocity travels a shorter distance along the scan axis than a drop having nominal drop velocity, and strikes the print medium sooner, whereby the actual drop location will be displaced from the location at which a drop of nominal drop velocity would strike the print medium (a target or nominal drop location), in a direction opposite the relative scan direction as referenced to the print medium. A drop having a lower than nominal drop velocity will have a longer flight time than a drop of nominal drop velocity, and will strike the print medium later than a drop having a nominal drop velocity, whereby the actual drop location will be displaced from the location at which a drop of nominal drop velocity would strike the print medium, along the scan direction as referenced to the print medium.
As a result of manufacturing tolerances, drop velocity varies from printhead to printhead, while for any given example of a particular printhead drop velocity is reasonably constant for all of the nozzles thereof. In unidirectional printing wherein the direction of relative motion between the nozzles and the print medium while printing is always the same, variation in drop velocity from nominal does not affect print quality since the displacement of actual drop location from nominal will be substantially the same for each scan. However, in bidirectional printing wherein the nozzles are reciprocatingly scanned relative to the print media while printing, variation in drop velocity from nominal affects print quality, since the displacements from nominal of the actual drop placements depend on the scan direction. A straightforward manifestation of bidirectional misalignment is the non-colinearity of alternating segments of a vertical line, wherein the alternating segments are printed in opposing scan directions.
Bidirectional alignment is commonly achieved by shifting the print data along the carriage axis for one of the opposing scan directions, in increments of the carriage axis dot resolution, for example. The amount of print data shifts is determined for example by printing test patterns that include line segments that extend orthogonally to the carriage axis and are printed in opposite scan directions with different data shifts for a predetermined one of the opposing scan directions, and identifying the best pattern, either visually or opto-electronically.
While ink jet printers commonly include provisions for bidirectional alignment, a user might not perform such alignment when appropriate, for example after installation of a new ink jet cartridge. Accordingly, some printers use only unidirectional printing for certain high quality print modes or print modes that involve media that are susceptible to image quality degradation due to bidirectional misalignment.
There is therefore a need for adaptively utilizing bidirectional printing to increase throughput.